-I couldn’t leave my best girl.Not when she owes me a dance.
What makes this part such a punch to the gut is the fact that the beginning of the scene itself isn’t actually sad at all. Sure, there’s a wistfulness on Steve’s side and he’s quite frank about feeling lost in this new world, but Peggy is honest with him in return, like she has always been and she also makes it clear that she has no regrets with how she had lived her life. This is how their relationship has always been defined, and Steve is simply happy for her, so proud of her. She is his anchor, his compass pointing north, and it doesn’t matter whether she’s 25 or 95.
Then in a split second, she forgets everything, forgets the present him. The him that was just smiling with her just moments ago. In her own fight against such a cruel disease, Peggy only remembers him in terms of her grief of losing him 70 years ago. And that’s why Steve crumbles so visibly here, even as he tries so hard to hold himself together and be brave for her. It’s what Peggy herself would’ve done if their situations have been reversed, and Steve knows it.
And arguably, it’s what Peggy did when she spoke to Steve on the radio. It was heartrending and terrifying, but Peggy stayed on with him until it cut out. What’s more, she gave him what he needed then— a light distraction from his demise, a promise, a voice that he loved in his ear.
When Peggy said that he wouldn’t be alone, she didn’t just mean bodies with him on Hydra raids, or even a voice on the radio— she meant that he would be supported. Steve is honoring their accord. He will put on his brave face, despite how brokenhearted he is to see the change in her demeanor, and he will tell Peggy the truth: that he couldn’t leave his best girl.